Radiation sensitive intrusion alarm systems have received substantial interest in recent years as a result of the increase in the number of burglaries committed in the United States and various foreign countries. Such interest has been further enhanced with the advent of significant improvements made in the responsivity of infrared radiation detectors operating in the 8-15 micron wavelength range. Any object having a temperature above absolute zero emits radiation in the infrared wavelength region of the electromagnetic radiation spectrum, and the 8-15 micron wavelength range is of particular interest as a "window" in the well-known transmission spectrum for infrared radiation.
Highly sensitive thermal detectors which are responsive to 8-15 micron wavelength infrared radiation have been developed and generate a usuable electrical output signal in response to changes in levels of infrared radiation received. As a result there have been various designs proposed for electro-optical intrusion alarm systems utilizing these detectors and operative to generate numerous selected fields of view over certain areas under surveillance.
Some of the earlier electro-optical intrusion alarm systems of the above type include those which generate a field of view covering the total desired space (volume) under surveillance. When an intruder penetrates any location within this space, he creates a minute change in the total average radiation seen by the system's field of view, and this change in radiation in related to a temperature differential caused by the intruder's average temperature relative to the average background temperature of the space penetrated. This temperature differential is usually very small, i.e. less than about 1.degree.C., and must be detected by the radiation sensor portion of the electro-optical intrusion alarm system. Since this level of sensitivity required for IR detection is constant, one solution to providing a dependable false alarm-free system with a high signal-to-noise ratio is to reduce the overall noise level in the background radiation.